1. Field of the Invention
The present invention relates to wireless communication systems and, in particular, to a wireless communication system using multicarrier transmission technique such as Orthogonal Frequency-Division Multiplexing (OFDM) or Code Division Multiplexing (CDM).
More particularly, the invention relates to a wireless communication system and method in which the problem of reducing the Peak to Average Power Ratio (PAPR) can be overcome.
2. Description of the Relevant Art
The multicarrier transmission technique known as orthogonal frequency-division multiplexing (OFDM) has been adopted in many recent wireless communications standards. For example, the IEEE 802.11a specifications for WiFi are based on 64-carrier OFDM and the most popular mode of IEEE 802.16 specifications is based on 256-carrier OFDM. This technology also appears today as one of the strongest candidates for cellular communications after the third-generation cellular communication systems.
One of the major problems of the OFDM signal format is the large peak-to-average power ratio (PAPR). Indeed, even with simple signal constellations like the quaternary phase-shift keying (QPSK) constellation used in many systems, OFDM leads to high peak power values, because the transmitted signal is the sum of a large number of uncorrelated signals and its amplitude distribution looks like Gaussian. The consequence of this is that the power amplifier at the transmitter must be substantially backed off from its saturation point so as to operate in its linear region in order not to distort the OFDM signal. Therefore, power amplifiers with a large linear range are required for OFDM systems, but such amplifiers are major cost components of the OFDM systems. Besides, if the amplifier back off is not sufficient, its nonlinear characteristics will degrade the bit error rate (BER) performance, on one hand, and affect the spectrum shaping of the transmitted signal, on the other hand. As a result of the second effect, the transmitted signal spectrum may lose its compliance with the applicable spectral mask.
The same problem also arises in code-division multiplexing (CDM), code-division multiple access (CDMA), and in other transform-based digital signal formats. In a cellular system based on CDMA, for example, the signal transmitted by the base station (BS) is a sum of signals spread using different spreading codes or sequences, and its PAPR is similar to that of OFDM. On the uplink too, namely from users to the base station, the transmitted signal is often a sum of spread signals, because a multitude of spreading codes are assigned to users requesting a high data rate on the uplink. For this reason, the high PAPR problem is also encountered on both the uplink and the downlink of CDMA-based systems. This is the case in the third-generation cellular standards including the Universal Mobile Telecommunications Standard (UMTS) adopted in Europe.
Reference will now be made to FIG. 1 which schematically shows the variation of the output power Pout of a power amplifier of transmitter as a function of the input power Pin, illustrating the operating regions of the amplifier. As previously indicated, the most common approach to handle power amplifier nonlinearities is to sufficiently back off the power amplifier from its saturation point such that the amplifier operates in its linear region. However this solution is very costly as it requires the use of transmit amplifiers with a very high saturation power and therefore a large linear range. If the peak power Ppeak of the transmitted signal is in the linear region of the amplifier and its peak-to-average power ratio (PAPR) is large, then the average transmitted signal power Pave, will be very far from the saturation power Psat, and obviously the amplifier will be used in a very inefficient way.
Another solution consists in clipping the input signal to the power amplifier so that the clipped signal amplitude essentially lies in the amplifier's linear region. This technique makes the signal insensitive to the amplifier nonlinearity, but in this case, the clipping operation itself leads to signal distortion and some spectral widening.
Other well-known techniques to handle nonlinear distortion are predistortion at the transmitter and nonlinear channel equalization at the receiver. Since channel equalization is placed at the receiver, it does not change the shape of the transmitted signal and does not help with respect to spectrum widening. As for predistortion techniques, they need to be adaptive to track the amplifier response changes and this involves a substantial complexity.
The foregoing techniques are applicable to all digital signal formats. There are also some techniques that are specific to OFDM communication systems. These aim at reducing the peak power or the PAPR of OFDM signals. Peak power reduction techniques in OFDM include coding, phase optimization and multiple signal representation.
Reference can be made to J. Tellado, Multicarrier Modulation with Low Peak to Average Power Applications to xDSL and Broadband Wireless, Boston/Dordrecht/London: Kluwer Academic Publishers, 2000.
As concerns coding technique, this technique consists essentially in excluding the code words with high PAPR. This technique is for example disclosed in K. Patterson, “Generalized Reed-Muller Codes and Power Control in OFDM Modulation,” IEEE Transactions on Information Theory, vol. 46, pp. 104-120, January 2000.
As an alternative, PAPR reduction can be achieved using a phase optimization, as disclosed in C. Tellambura, “Phase Optimization Criterion for Reducing Peak-to-Average Power Ratio in OFDM,” Electronics Letters, vol. 34, pp. 169-170, January 1998 or using schemes relying on multiple signal representation, namely Selective Maping (SLM) and Partial Transmit Sequences (PTS) algorithms as disclosed in M. Breiling, S. H. Muller, and J. B. Huber, “SLM Peak-Power Reduction without Explicit Side Information,” IEEE Communications Letters, vol. 5, pp. 239-241, June 2001; and in S. H. Muller and J. B. Huber, “OFDM with Reduced Peak-to-Average Power Ratio by Optimum Combination of Partial Transmit Sequences,” Electronics Letters, vol. 33, pp. 368-369, February 1997.
As concerns Partial Transmit Sequences, each block of a subcarrier is multiplied by constant phase factors, and these phase factors are optimized to minimize the PAPR. In selective maping, multiple sequences are generated from the same information, and a sequence with the lowest PAPR is transmitted by the transmitter.
However, the main problem of these techniques is that they require a transmission of side information to the receiver such that the receiver receives information concerning the treatment carried out by the transmitter onto the transmitted data.
At last, more recent attempts reduce the peak power by using a constellation shaping in a signal constellation, namely by changing the signal constellation, introducing new constellations, or inserting pilot signals either in unused subcarriers or over some or all of the used carriers, as indicated in J. Tellado, “Multicarrier Modulation with Low Peak to Average Power Applications to xDSL and Broadband Wireless”, Boston/Dordrecht/London: Kluwer Academic Publishers, 2000.
These techniques, however, involve very complex optimization procedures.